The present invention comprises a method for forming a stable, polymeric emulsifier-based, water-in-oil emulsion phase having improved stability following homogenization.
Water-in-oil emulsion explosives, hereafter termed xe2x80x9cemulsion explosives,xe2x80x9d are well-known in the industry. They comprise an emulsified dispersion of a discontinuous phase of inorganic oxidizer salt solution droplets in a continuous organic fuel phase. This dispersion or emulsion phase is held in place by a water-in-oil emulsifier (hereafter xe2x80x9cemulsifierxe2x80x9d) provided the emulsified state remains stable. The inorganic oxidizer salt solution droplets typically are in a super-cooled state and thus want to crystallize and consequently destabilize the emulsified state. Thus if the emulsified state weakens, the emulsion will destabilize and the salts in the droplets will crystallize, causing further destabilization. This crystallization desensitizes the emulsion explosive and can render it undetonable.
Destabilization is a common problem when the emulsion explosive is subjected to xe2x80x9cworkingxe2x80x9d or is xe2x80x9cworked,xe2x80x9d which means to subject the emulsion phase to shearing action such as when the emulsion phase is pumped or otherwise transferred into a borehole or other container or is mixed with additional ingredients such as sensitizing microballoons or AN prills. In explosives applications, an emulsion phase is commonly subjected to working in this fashion and thus the emulsion phase must be able to retain its stability even after working. The formulations disclosed herein have greater stability when subjected to these normal processing and handling conditions. In addition to these common transfer and mixing operations, an emulsion phase may purposely be subjected to very high shear conditions by various means in order to increase the viscosity of the emulsion phase. This process is commonly (and herein) referred to as homogenization. As homogenization occurs, the dispersed oxidizer salt solution droplets become smaller in size and consequently the viscosity of the emulsion phase increases. This viscosity increase oftentimes is desirable because it enables the emulsion explosive to resist water intrusion, retain its stability and remain in the borehole rather than flowing out of an upwardly extending borehole or into cracks or fissures. Along with the viscosity increase and smaller solution droplet size that result from homogenization, however, the propensity of the emulsion phase to experience crystallization increases under such high shear conditions. Thus, for a given composition there is a practical limit to the degree of homogenization that can occur before crystallization becomes unacceptable.
Although polymeric emulsifiers, such as those based on various adducts of polyisobutenyl succinic anhydride (xe2x80x9cPIBSAxe2x80x9d), are found to form stable emulsion phases under certain conditions, emulsion phases containing polymeric emulsifiers tend to destabilize upon homogenization. Efforts at inhibiting such destabilization include replacing a portion of the polymeric emulsifier with nonpolymeric emulsifiers that are less susceptible to homogenization destabilization such as sorbitan monooleate (SMO). The nonpolymeric emulsifiers, however, tend to form emulsion phases that are less stable with time than those formed with primarily or solely polymeric emulsifiers, both before and after homogenization. Thus, where mixtures of polymeric and nonpolymeric emulsifiers were used, both stability and homogenizeability were compromised to a degree. The present invention allows an emulsion phase to be formed that is both stable and homogenizeable.
One method for homogenizing emulsion explosives is disclosed in U.S. Pat. No. 4,615,752, which involves positioning a valve at the end of a delivery hose in order to increase the viscosity of the explosive through the shearing action of the valve. In addition, in-line mixing devices to impart high shear to an emulsion during flow (pumping) are used in the industry, and these can be positioned anywhere within the delivery train of the emulsion. Another method for improving homogenization is disclosed in U.S. Pat. No. 5,322,576, which discloses replacing at least a portion of the organic fuel phase with a vegetable oil.
The method of the present invention for forming a stable, polymeric emulsifier-based emulsion phase following homogenization comprises:
(a) forming an inorganic oxidizer salt solution,
(b) forming an organic fuel phase which comprises at least about 3% by weight of the fuel phase of an homogenization additive selected from the group consisting of animal oils and fatty acids,
(c) mixing the organic fuel phase and the inorganic oxidizer salt solution phase in the presence of a polymeric emulsifier with sufficient shear to form the emulsion phase, and then,
(d) homogenizing the emulsion phase to increase its viscosity prior to placement or packaging of the product.
The use of an homogenization additive selected from the group consisting of animal oils and fatty acids in an amount of at least about 3% by weight of the organic fuel phase has been found to improve the long-term stability of a homogenized emulsion phase that contains a polymeric emulsifier. In test results shown in the tables below, this stability improvement surprisingly is better than that provided by an organic fuel phase that contains a vegetable oil.